Not sure what case this is, but a fix none-the-less.

taggit/forms.py

@@ -5,7 +5,7 @@ from taggit.utils import parse_tags
 
 class TagWidget(forms.TextInput):
     def render(self, name, value, attrs=None):
-        if not isinstance(value, basestring):
+        if value is not None and not isinstance(value, basestring):
             value = ", ".join(o.tag.name for o in value.select_related("tag"))
         return super(TagWidget, self).render(name, value, attrs)